1. Field of the Invention
The present invention relates to an electronic circuit that has a balanced output and an unbalanced output together, and a television tuner having such an electronic circuit.
2. Description of the Related Art
In the related art, as an input method to a mixing circuit having a differential circuit, there are known a method that directly inputs balanced input signals to differential input terminals (for example, see JP-A-2003-51860) and a method that grounds one differential input terminal so as to use the differential input terminal as an unbalanced input terminal (for example, see JP-A-2003-309777). As an electronic circuit that has a balanced output and an unbalanced output together, a UHF-VHF television tuner is known.
FIG. 7 is a block diagram showing the configuration of a television tuner according to the related art. FIG. 7 shows the configuration from an antenna to an intermediate-frequency amplifying circuit. A television tuner 100 shown in FIG. 7 includes a UHF-side block 101 and a VHF-side block 102. The UHF-side block 101 includes an antenna tuning circuit 111, a high-frequency amplifying circuit 112, an interstage tuning circuit 113, and a mixing circuit 114. All the elements are sequentially connected in a cascade manner. Similarly, the VHF-side block 102 includes an antenna tuning circuit 121, a high-frequency amplifying circuit 122, an interstage tuning circuit 123, and a mixing circuit 124. All the elements are sequentially connected in a cascade manner.
In the VHF-side block 102, a high-band or low-band high-frequency signal of a VHF band is extracted by the antenna tuning circuit 121, and the extracted signal is amplified by the high-frequency amplifying circuit 122. A desired television signal is selected according to a tuning voltage corresponding to channel-selection data by the interstage tuning circuit 123, and the selected television signal is output to the mixing circuit 124 in an unbalance manner.
Here, the interstage tuning circuit 113 of the VHF-side block 102 is an unbalanced circuit of an unbalanced output type. The interstage tuning circuit 113 is coupled to only one input terminal of the mixing circuit 124 having a differential circuit, and the other input terminal thereof is fixedly grounded. The mixing circuit 124 mixes a local oscillation signal, which is input from a VHF high-band oscillation circuit 125 or a VHF low-band oscillation circuit 126, and the selected television signal, and performs frequency conversion of the mixed signal into an intermediate-frequency signal. The intermediate-frequency signal of the selected television signal is extracted by an intermediate-frequency tuning circuit 131, is amplified by an intermediate-frequency amplifier 132, and is input to a rear-stage circuit (not shown).
In the UHF-side block 101, a television signal of a UHF band is extracted by the antenna tuning circuit 111 and is amplified by the high-frequency amplifying circuit 112. Then, a desired television signal is selected according to a tuning voltage corresponding to channel-selection data by the interstage tuning circuit 113, and the selected television signal is output to the mixing circuit 114 in a balanced manner.
Here, the interstage tuning circuit 113 of the UHF-side block 101 is a balanced circuit of a balanced output type. The interstage tuning circuit 113 is coupled to both input terminals of the mixing circuit 114 having a differential circuit. The mixing circuit 114 mixes a local oscillation signal, which is input from a UHF oscillation circuit 127, and the selected television signal, and performs frequency conversion of the mixed signal into an intermediate-frequency signal. The intermediate-frequency signal of the selected television signal is extracted by the intermediate-frequency tuning circuit 131, is amplified by the intermediate-frequency amplifier 132, and is input to the rear-stage circuit (not shown).
However, according to this configuration, since the unbalanced input type mixing circuit 124 corresponding to the unbalanced output of the interstage tuning circuit 123 of the VHF-side block 2, and the balanced input type mixing circuit 114 corresponding to the balanced output of the interstage tuning circuit 113 of the UHF-side block 101 are separately provided, costs are increased and the size of the circuit is increased. In particular, when a portion 140 (the mixing circuits and the local oscillation circuits) surrounded by a dotted line is integrated as an integrated circuit, it is difficult to reduce the size because the two mixing circuits 114 and 124 exist together.
Moreover, in the related art, the balanced input type mixing circuit 114 is provided to correspond to the balanced output of the interstage tuning circuit 113 of the UHF-side block 101, and the unbalanced input type mixing circuit 124 is provided to correspond to the unbalanced output of the interstage tuning circuit 123 of the VHF-side block 102. Two front-stage circuits of an unbalanced output type and a balanced output type are provided. Then, as for rear-stage circuits, for example, upon amplification of a high-frequency amplifying circuit, two types of differential amplifying circuits, that is, an unbalanced input type differential amplifying circuit and a balanced input type differential amplifying circuit are also provided. In this case, costs and the size of the circuit are increased.